Nitride semiconductor device

ABSTRACT

An nitride semiconductor device for the improvement of lower operational voltage or increased emitting output, comprises an active layer comprising quantum well layer or layers and barrier layer or layers between n-type nitride. semiconductor layers and p-type nitride semiconductor layers, wherein said quantum layer in said active layer comprises InxGa1−xN (0&lt;x&lt;1) having a peak wavelength of 450 to 540 nm and said active layer comprises laminating layers of 9 to 13, in which at most 3 layers from the side of said n-type nitride semiconductor layers are doped with an n-type impurity selected from the group consisting of Si, Ge and Sn in a range of 5×10 16  to 2×10 18 /cm 3 .

BACKGROUND OF THE INVENTION

This invention relates to a nitride semiconductor device used for lightemitting devices such as a light emitting diode (LED) and a laser diode(LD), light receiving devices such as a solar cell and a light sensorand electronic devices such as a transistor and a power device,especially relates to an improved quantum well structure light emittingdiode having an emitting peak wave length in a range of 450 to 540 nmwherein a loared operating voltage and an increased output can beobtained.

Nitride semiconductors have been used to make high bright and pure greenand blue LEDs for full color displays, traffic signals and light sourcesfor image scanner and so on. These LED devices are made of laminatedstructures which basically comprise a GaN buffer layer formed on asapphire substrate, a n-type GaN contact layer doped with Si, ansingle-quantum-well (SQW) or multi-quantum-well (MQW) active layercomprising InGaN, a p-type AlGaN clad layer doped with Mg and a p-typeGaN contact layer doped with Mg. The SQW blue laser having a peak wavelength of 470 nm has shown a very superior characteristic such as theoutput of 2.5 mW and the external quantum efficiency of 5% at 20 mA,whereas the MQW has shown the output of 5 mW and the external quantumefficiency of 9.1% at 20 mW. Further, the SQW blue. laser having a peakwave length of 520 nm has shown the output of 2.2 mW and the externalquantum efficiency of 4.3% at 20 mA, whereas the MQW has shown theoutput of 3 mW and the external quantum efficiency of 6.3% at 20 mW.

The MQW are expected to get an improved device characteristic such ashigher outputs as compared to the SQW because the MQW can emit the lightefficiently at a small current due to a plurality of mini-bandstructures. As a typical LED device having the MQW active layer forgetting a good efficiency and output, Japanese Patent Kokai Hei10-135514 discloses a nitride semiconductor light emitting device whichcomprises a MQW light emitting layer comprising laminated structures ofundoped GaN barrier layers and undoped InGaN quantum well layers betweenclad layers having a wider band gap than that of the barrier layer.However, in order to improve the output of the blue green LED having alonger peak wavelength, there is proposed the increased number of layersin the MQW structure. The forward voltage Vf becomes higher depending onthe layer number of MQW, resulting in such problems as the higherforward voltage Vf and the lowered emitting output.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a nitridesemiconductor device having an active layer of a quantum well structurewith large number of layers and relatively low forward voltage,especially with an improved emitting efficiency and a higher emittingoutput.

As a result of focused research for luminous phenomenon in the lightemitting diode having a Multi Quantum Well (MQW) structure betweenn-type semiconductor layers and p-type semiconductor layers, there wasfound that recombination of electrons and holes in the MQW active layermainly happen in a quantum well layer or layers close or proximate tothe p-type nitride semiconductor layers and rarely happen in a quantumwell layer close or proximate to the n-type nitride semiconductorlayers. That is, the quantum well layer near to the n-type nitridesemiconductor layers can hardly function as the emitting layer. Afterthat, there was found that, when an n-type impurity is doped into thequantum well layer close to the n-type nitride semiconductor layers, thecarrier density thereof increases, so that the forward voltage can belowered and the emitting efficiency can be improved. The presentinvention was completed on the basis of the above finding. According toa first aspect of the present invention, there is provided a nitridesemiconductor device which comprises an active layer containing ann-type impurity and comprising a quantum well layer or layers and abarrier layer or layers between n-type nitride semiconductor layers andp-type nitride semiconductor layers, wherein at least one of saidbarrier layers and said quantum well layers at the proximate side insaid active layer to said n-type nitride semiconductor layers is dopewith n-type impurity. In this invention, the donor can be additionallysupplied into the active layer from the n-type nitride semiconductorlayers, so that a higher output can be obtained. In a preferredembodiment of the present invention, the layers to be doped with then-type impurity should be determined according to the following formula(1). Because, if the number of the doped layers are beyond thedetermined number, a good output can not be obtained due to thedeterioration of crystal quality. If said active layer is a MQW having(i) laminated layers, then at least one of 1st to j-th layers countingfrom the side proximate to said n-type nitride semiconductor layers isdoped with n-type impurity, wherein j′=i/6 +2, where i is an integer ofat least 4, and wherein j is the integer portion of j′.

In the present invention, the layer doped with the n-type impurity meansthe layer intentionally doped with the n-type impurity, preferably in arange of 5×10¹⁶ to 2×10¹⁸/cm³. In a case that the layer contains then-type impurity in a range of 5×10¹⁶ to 2×10¹⁸/cm³ due to the diffusionof the n-type impurity from the neighboring layer and the contaminationfrom original materials and CVD devices, the unintentional doping layeralso belongs to the doped layer.

Generally, the barrier layer and/or the quantum well layer is preferablyan undoped layer for functioning as the emitting layer. In the presentinvention, the undoped layer means a layer not containing the n-typeimpurity of more than 5×10¹⁶/cm³.

In a preferred embodiment of the present nitride semiconductor device,the barrier layer and/or the quantum well layer at the distal side tosaid n-type semiconductor layers may be not doped with n-type impurity.Therefore, in the preferred nitride semiconductor device having theactive layer of a SQW, the quantum well layer and the barrier layer atthe proximate side to said p-type semiconductor layers are not dopedwith n-type impurity. On the other hand, in a case of MQW the proximatelayers to said n-type semiconductor layers may be doped with n-typeimpurity whereas said proximate layers to said p-type semiconductorlayers may not be doped with n-type impurity.

In a preferred case, said active layer comprises 9 to 15 layers, at most4, preferably at most 3 layers of which from said n-type semiconductorlayers are doped with n-type impurity.

The above structure may be applied to the active layer comprisesInxGa1−xN (0<x<1) suited to the emitting light of 450 to 540 nm,preferably 490 to 510 nm.

In a preferred embodiment, the n-type impurity may be selected from thegroup consisting of Si, Ge and Sn. The n-type impurity content of theactive layer may be lower than that of said n-type semiconductor layers.In the other case, the n-type impurity content of the active layer maydecrease depending on distance from said n-type semiconductor layers.Generally, the n-type impurity content of the active layer may be in arange of 5×10¹⁶ to 2×10¹⁸/cm³. Preferably the n-type impurity content ofthe barrier layer and/or the quantum well layer in the active layer maybe in a range of 5×10¹⁶ to 2×10¹⁸/cm³.

In a typical case, the n-type impurity content of the barrier layer isin a range of 5×10¹⁶ to 2×10¹⁸/cm³, whereas the n-type impurity contentof the quantum well layer is in a range of 5×10¹⁶ to 2×10¹⁸/cm³ andlower than that of the barrier layer. In another typical case, then-type impurity content of the barrier layer is in a range of 5×10¹⁶ to2×10¹⁸/cm³, whereas the n-type impurity content of the quantum welllayer in the active layer is less than 5×10¹⁶ to 2×10¹⁸/cm³ and lowerthan that of said barrier layer.

In the present invention, for the improvement of higher output, thethickness of the barrier layer or quantum well layer close or proximateto said n-type semiconductor layers is larger than that of said barrierlayer or quantum well layer close or proximate to said p-typesemiconductor layers. For the improvement of low operational voltage,the thickness of the barrier layer or quantum well layer close orproximate to the n-type semiconductor layers is smaller than that of thebarrier layer or quantum well layer close or proximate to the p-typesemiconductor layers.

The inventive MQW structure can be preferably applied to a blue-greenlight emitting diode. Therefore, according to a second aspect of thepresent invention, there can be provided a nitride semiconductoremitting device which comprises an active layer comprising quantum welllayer or layers and barrier layer or layers between n-type nitridesemiconductor layers and p-type nitride semiconductor layers, whereinthe quantum layer in the active layer comprises InxGa1−xN (0<x<1) havinga peak wavelength of 450 to 540 nm and the active layer compriseslaminating layers of 9 to 13, in which at most 3 layers from the side ofthe n-type nitride semiconductor layers are doped with an n-typeimpurity selected from the group consisting of Si, Ge and Sn at a rangeof 5×10⁶ to 2×10¹⁸/cm³.

In a typical embodiment of the present invention, the thickness of thebarrier layer or quantum well layer close or proximate to the n-typesemiconductor layers is larger than that of the barrier layer or quantumwell layer close or proximate to the p-type semiconductor layers. Inanother typical embodiment of the present invention, the thickness ofthe barrier layer or quantum well layer close or proximate to the n-typesemiconductor layers is smaller than that of the barrier layer orquantum well layer close or proximate to the p-type semiconductorlayers.

In a preferred embodiment, the inventive MQW active layer can bepreferably applied to the light emitting diode having the quantum welllayer of InxGa1−xN (0<x<1) having a peak wavelength of 490 to 510 nm. Inthis case, the barrier layer may comprise InyGa1−yN (0≦y<1, y<x).

In a more preferred embodiment, the active layer comprising an MQW ofInxGa1−xN (0<x<1)/InyGa1−yN (0≦y<1, y<x) lamination may be formed on ann-type multi-layer, which may be selected from the group consisting of abuffer super lattice layer undoped with n-type impurity and comprisingInzGa1−zN (0<z<1)/GaN lamination or AlwGa1−wN (0<w<1)/ GaN lamination.In this case, the GaN layer of the buffer super lattice layer may have athickness of less than 70 Å whereas the barrier layer of the activelayer may have a thickness of more than 70 Å.

In another preferred embodiment, the multi-layer may be doped withn-impurity and comprises lamination of GaN layer and a layer selectedfrom the group consisting of InzGa1−zN (0<z<1, z<y) layer having alarger band gap energy than that of the quantum well layer and AlwGawN(0<w<1) layer. In this case, the n-type impurity for doping into theactive layer and the n-type clad layer is preferably Si and the Sicontent of the active layer may be in a range of 5×10¹⁶ to 2×10¹⁸/cm³whereas the Si content of said n-clad layer may be in a range of morethan 5×10¹⁷/cm³ and larger than that of the active layer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objectives and features of the present inventionwill become more apparent from the following description of a preferredembodiment thereof with reference to the accompanying drawings,throughout which like parts are designated by like reference numerals,and wherein:

FIG. 1 is a schematic cross-sectional view of the preferred embodimentof the LED device of the present invention; and

FIG. 2 is a schematic cross-sectional view of an example of a MQWstructure of the LED device shown in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

This application is based on application No.11-159482 filed Jun. 7, 1999in Japan, the content of which is incorporated hereto by reference.

FIG. 1 is a schematic cross-sectional view showing the structure of thenitride semiconductor device according to an embodiment of the presentinvention. The present invention will be described in detail withreference to FIG. 1.

FIG. 1 shows an nitride semiconductor device in such a structure that abuffer layer 2, an undoped GaN layer 3, an n-type contact layer 4 madeof Si doped GaN, a first n-type multi-layered film 5, a second n-typemulti-layered film 6, an active layer 7 in the multi-quantum-wellstructure made of InGaN/GaN a p-type multi-layered film 8 and a p-typecontact layer 9 made of Mg doped GaN are laminated sequentially on thesubstrate 1. The composition of each layer and/or the number oflaminated layers are different between the n-type multi-layered film 6and the p-type multi-layered film 8.

In an embodiment of the present invention, the active layer is in themulti-quantum well structure having a multi-layered structure formed bylaminating well layers and barrier layers sequentially. The minimumlayered structure can be a three-layered structure which has a barrierlayer and two well layers provided on either side of the barrier layeror which has a well layer and two barrier layers provided on either sideof the well layer. In the multi-quantum-well structure, the twooutermost layers are constituted by well layers or barrier layers,respectively, but one outermost layer may be a well layer and the otherouter most layer may be a barrier layer. In the multi-quantum-welllayer, the last layer in the p-type layer region may be a barrier layeror a well layer.

For the active layer in such a multi-quantum-well structure, both welland barrier layers may be made of nitride semiconductor containingindium and gallium (preferably InGaN). But the well layer may be made ofnitride semiconductor containing indium and gallium (preferably InGaN)or GaN, and the barrier layer may be made of, for example, AlN or GaN.For example, the well layer of the active layer in themulti-quantum-well structure is made of a nitride semiconductorcontaining at least In, preferably In_(x)Ga_(1−x)N (0≦X<1). While thebarrier layer may be made of a nitride semiconductor having a band gapenergy larger than that of the well layer, preferably In_(y)Ga_(1−Y)N(0≦Y<1, X>Y) or Al₂Ga_(1−Z)N (0<Z<0.5)

An n-type impurity doped in the active layer may be selected from IVGroup elements such as Si, Ge, Sn, S, O, Ti or Zr, or VI Group elements,preferably may be Si Ge or Sn, most preferably Si.

According to the present invention, the concentration of the n-typeimpurity in the active layer is larger in the n-type layer region thanin p-type layer region. More preferably, the layers which meet theabove-mentioned equation (1) on the basis of the layer close to then-type nitride semiconductor layer are doped with an n-type impurity.The expression that the concentration of the n-type impurity in then-type layer region is larger than in the p-type layer region means, forexample, the case that in the active layer in the multi-quantum-wellstructure formed by laminating the well layer and the barrier layeralternately, in 11 layers in all, six layers in the n-type layer regionare doped with an n-type impurity and the remaining 5 layers in thep-type layer region are not doped with an n-type impurity. Also it meansthat in such a case, only well layers among 6 layers in the n-type layerregion are doped with an n-type impurity. The number of layers and dopedlayers may be varied provided that the n-type layer region is doped withan n-type impurity in the larger concentration.

According to the present invention, the total thickness of the activelayer is not particularly specified. But the total thickness is the sumof the thickness of well layers and barrier layers and is, for example,500 to 5000 angstroms, preferably, 1000 to 3000 angstroms. The totalthickness of the active layer is preferably within the above-mentionedrange in the term of the light output power and the time required forthe crystal growth of the active layer.

The single thickness of the barrier layer which constitutes themulti-quantum-well structure of the active layer is 70 to 500 angstroms,preferably 100 to 300 angstroms. The single thickness of the barrierlayer is preferably within the above-mentioned range, with the resultthat the photoelectric transfer efficiency is enhanced and Vf and theleak current are decreased.

The single thickness of the well layer of the active layer is not morethan 100 angstroms, preferably not more than 70 angstroms, morepreferably not more than 50 angstroms. The lower limit of the singlethickness of the well layer is not particularly specified, and it ispreferably not less than 10 angstroms. The single thickness of the welllayer is preferably within the above-mentioned range, with the resultthat the light output power is increased and the half band width of theemission spectrum is decreased.

The concentration of an n-type impurity doped in the active layer iscontrolled to be not more than the amount of Si doped in the n-typecontact layer, preferably 5×10¹⁶/cm³ to 1×10¹⁹/cm³, more preferably5×10¹⁶/cm³ to 5×10¹⁸/cm³, most preferably 5×10¹⁶/cm³ to 2×10¹⁸/cm³, morepreferably. The concentration of an n-type impurity is preferably withinthe above-mentioned range, with the result that Vf can be decreasedwithout the decrease of the photoelectric transfer efficiency and theincrease of the leak current in the I-V characteristics.

According to the present invention, the device structure except theactive layer is not particularly specified and various structures can beutilized. The concrete embodiment of the device structure may include,for example, one that will be described in the following examples. Theelectrode is also not particularly specified and various electrodes canbe utilized.

EXAMPLES

The examples according to an embodiment of the present invention will bedescribed in the following part. But the present invention is notlimited to those examples.

Example 1

Example 1 will be described with reference to FIG. 1 and FIG. 2.

(Substrate 1)

A C-face sapphire substrate 1 is set in the MOVPE reactor and thetemperature of the substrate is increased to 1050° C. with hydrogenflown to clean the substrate. The substrate 1 may be a R-face or A-facesapphire substrate, an insulting substrate like spinel (MgAl₂O₃), or asemiconductor substrate such as SiC (including 6H, 4H and 3C), Si, ZnO,GaAs, GaN and the like.

(Buffer Layer 2)

Subsequently, the temperature is decreased to 510° C. A buffer layer 2made of GaN is grown to a thickness of about 200 angstroms on thesubstrate 1 using hydrogen as a carrier gas and ammonia (NH₃) andtrimethylgallium (TMG) as a source gas. The first buffer layer 2 whichis grown at the low temperature may be omitted depending on the kind ofthe substrate and the growth method. The buffer layer may be made ofAlGaN having a small mixing proportion of Al.

(First Undoped GaN Layer 3)

After growing the buffer layer 2, only TMG is stopped and thetemperature is increased to 1050° C . At 1050° C., likewise usingammonia and TMG as a source gas, a first undoped GaN layer 3 is grown tothe thickness of 1 μm.

(n-Type Contact Layer 4)

Subsequently, at 1050° C., likewise using TMG and ammonia as a sourcegas and silane gas as an impurity gas, an n-type contact layer made ofGaN doped with Si to 3×10¹⁹/cm³ is grown to the thickness of 2.165 μm.

(n-Type First Multi-layered Film 5)

Next, only silane gas is stopped and at 1050° C., using TMG and ammoniagas, a lower layer 5 a made of undoped GaN is grown to the thickness of3000 angstroms. Subsequently, at the same temperature, the silane gas isadded and a middle layer 5 b made of GaN doped with Si to 4.5×10¹⁸/cm³is grown to the thickness of 300 angstroms. Further, subsequently, theonly silane gas is stopped and an upper layer 5 c made of undoped GaN isgrown to the thickness of 50 angstroms. Thus, an n-type multi-layeredfilm 5 which is constituted by 3 layers and had a total thickness of3350 angstroms is grown.

(n-Type Second Multi-layered Film 6)

Next, at the same temperature, a second nitride semiconductor layer madeof undoped GaN is grown to the thickness of 40 angstroms. Then, at 800°C., using TMG, TMI and ammonia, a first nitride semiconductor layer madeof undoped In_(0.13)Ga_(0.87)N is grown to the thickness of 20angstroms. These operations are repeated to laminate the layers in theorder of the second layer+the first layer, in 10 layers, respectively.Finally, the second nitride semiconductor layer made of GaN is grown tothe thickness of 40 angstroms. Thus, an n-type second multi-layered film6 in the super lattice structure is grown to the thickness of 640angstroms.

(Active Layer 7)

Next, a barrier layer made of undoped GaN is grown to. the thickness of200 angstroms using ammonia. Subsequently, at 800° C., a well layer madeof In_(0.3)Ga_(0.7)D₇N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms. Subsequently, at the same temperature, a well layermade of undoped In_(0.3)Ga_(0.7)N doped with Si to 5×10¹⁷/cm³ is grownto the thickness of 30 angstroms. Then, the barrier layers and the welllayers are laminated in the order of the barrier+well+barrier+ . . .+barrier layer. Thus, as illustrated in FIG. 2, 16 undoped barrierlayers and 15 well layers among which the initial 3 layers are dopedwith Si and the remaining 12 layers are undoped are laminatedalternately, in 31 layers in all, with the result that the active layer7 in the multi-quantum-well structure having a total thickness of 3650angstroms is obtained.

(p-Type Multi-layered Film 8)

Next, the temperature is raised to 1050° C. A third nitridesemiconductor layer made of Al_(0.2)Ga_(0,8)N doped with Mg to5×10¹⁹/cm³ is grown to the thickness of 40 angstroms using TMG, TMA,ammonia and Cp2Mg (cyclopentadienyl magnesium). Subsequently, at 800°C., a fourth nitride semiconductor layer made of In_(0.02)Ga_(0.98)Ndoped with Mg to 5×10¹⁹/cm³ is grown to the thickness of 25 angstromsusing TMG, TMI, ammonia and Cp2Mg. These operations are repeated tolaminate layers in the order of the third+fourth layer, in 5 layers,respectively. Finally the third nitride semiconductor layer is grown tothe thickness of 40 angstroms. Thus, a p-type multi-layered film 8 inthe super lattice structure having a total thickness of 365 angstroms isformed.

(p-Side Optical Waveguide Layer 11)

Next, Cp2Mg and TMA are stopped and 1050° C., a p-side optical waveguidelayer 11 made of undoped GaN and having a band gap energy lower thanthat of the p-side capping layer 10 is grown to the thickness of 0.1 μm.

This p-side optical guide layer 8 is undoped, that is, intentionallyundoped, but due to the diffusion of Mg from the adjacent p-side firstcladding layer and p-side second cladding layer, the real concentrationof Mg is 5×10¹⁶/cm³, resulting in the layer doped with Mg.

(p-Side Contact Layer 9)

Subsequently, at 1050° C., a p-type contact layer 8 made of p-type GaNdoped with Mg to 1×10²⁰/cm³ is grown to the thickness of 700 angstromsusing TMG, ammonia and Cp2Mg.

After the reaction is completed, the temperature is decreased to roomtemperature. Additionally, the wafer is annealed at 700° C. in nitrogenatmosphere within the reactor, so as to make the p-type layer lessresistive.

After annealing, the wafer is removed out of the reactor. A mask of apredetermined shape is formed on the surface of the uppermost p-typecontact layer 9 and etching is conducted from the p-type contact layerwith the RIE (reactive ion etching) apparatus, to expose the surface ofthe n-type contact layer 4, as shown in FIG. 1.

After etching, a translucent p-electrode 10 containing Ni and Au andhaving a thickness of 200 angstroms is formed on the almost entiresurface of the uppermost p-type contact layer. And an n-electrode 11containing W and Al is formed on the surface of the n-type contact layer4 which had been exposed by etching, resulting in a LED device.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.4V and the lightoutput power is 6.5 mW.

Example 2

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of undoped GaN is grown to the thickness of 200angstroms using TMG and ammonia. Subsequently, at 800° C., a well layermade of In_(0.03)Ga_(0.7)N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms and a well layer made of In_(0.3)Ga_(0.7)N doped withSi to 5×10¹⁷/cm³ is grown to the thickness of 30 angstroms. Then, thebarrier layers and the well layers are laminated in the order of thebarrier+well+barrier+ . . . +barrier layer. Thus, 11 undoped barrierlayers and 10 well layers among which the initial 2 layers are dopedwith Si and the remaining 8 layers are undoped are laminatedalternately, in 21 layers in all, with the result that the active layer7 in the multiquantum-well structure having a total thickness of 2500angstroms is obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.4V and the lightoutput power is 6.4 mW.

Example 3

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of undoped GaN is grown to the thickness of 200angstroms using TMG and ammonia. Subsequently, at 800° C., a well layermade of In_(0.3)Ga_(0.7)N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms and a well layer made of undoped In_(0.3)Ga_(0.7)N isgrown to the thickness of 30 angstroms. Then, the barrier layers and thewell layers are laminated in the order of the barrier+well+barrier+ . .. +barrier layer. Thus, 6 undoped barrier layers and 5 well layers amongwhich the initial layer is doped with Si and the remaining 4 layers areundoped are laminated alternately, in 11 layers in all, with the resultthat the active layer 7 in the multi-quantum-well structure having atotal thickness of 1350 angstroms is obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.4V and the lightoutput power is 6.3 mW.

Example 4

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of undoped GaN is grown to the thickness of 200angstroms using TMG and ammonia. Subsequently, at 800° C., a well layermade of In_(0.3)Ga_(0.7)N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms and a well layer made of undoped In_(0.3)Ga_(0.7)N isgrown to the thickness of 30 angstroms. Then, the barrier layers and thewell layers are laminated in the order of the barrier+well+barrier+ . .. +barrier layer. Thus, 3 undoped barrier layers and 2 well layers amongwhich the initial layer is doped with Si and the remaining layer isundoped are laminated alternately, in 5 layers in all, with the resultthat the active layer 7 in the multi-quantum-well structure having atotal thickness of 660 angstroms is obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.4V and the lightoutput power is 6.2 mW.

Example 5

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of GaN doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 200 angstroms using TMG, ammonia and silane gas.Subsequently, at 800° C., a well layer made of undoped In_(0.3)Ga_(0.7)Nis grown to the thickness of 30 angstroms using TMG, TMI and ammonia.Further, a barrier layer made of GaN doped with Si to 5×10¹⁷/cm³ isgrown to the thickness of 200 angstroms and a well layer made of undopedIn_(0.3)Ga_(0.7)N is grown to the thickness of 30 angstroms. Then, thebarrier layers and the well layers are laminated in the order of thebarrier+well+barrier+ . . . +barrier layer. Thus, 16 barrier layersamong which the initial 3 layers are doped with Si and the remaining 13layers are undoped and 15 undoped well layers are laminated alternately,in 31 layers in all, with the result that the active layer 7 in themulti-quantum-well structure having a total thickness of 3650 angstromsis obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.6V and the lightoutput power is 6.2 mW.

Example 6

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of GaN doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 200 angstroms using TMG, ammonia and silane gas.Subsequently, at 800° C., a well layer made of In_(0.3)Ga_(0.7)N dopedwith Si to 5×10¹⁷/cm³ is grown to the thickness of 30 angstroms usingTMG, TMI, ammonia and silane gas. Further, a barrier layer made of GaNdoped with Si to 5×10¹⁷/cm³ is grown to the thickness of 200 angstromsand a well layer made of In_(0.3)Ga_(0.7)N doped with Si to 5×10¹⁷/cm³is grown to the thickness of 30 angstroms. Then, the barrier layers andthe well layers are laminated in the order of the barrier+well+barrier+. . . +barrier layer. Thus, 16 barrier layers among which the initial 3layers are doped with Si and the remaining 13 layers are undoped and 15well layers among which the initial 3 layers are doped with Si and theremaining 12 layers are undoped are laminated alternately, in 31 layersin all, with the result that the active layer 7 in themulti-quantum-well structure having a total thickness of 3650 angstromsis obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.6V and the lightoutput power is 6.4 mW.

Example 7

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of GaN doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 200 angstroms using TMG, ammonia and silane gas.Subsequently, at 800° C., a well layer made of In_(0.3)Ga_(0.7)N dopedwith Si to 5×10¹⁷/cm³ is grown to the thickness of 30 angstroms usingTMG, TMI, ammonia and silane gas. Further, a barrier layer made of GaNdoped with Si to 5×10¹⁷/cm³ is grown to the thickness of 200 angstromsand a well layer made of In_(0.3)Ga_(0.7)N doped with Si to 5×10¹⁷/cm³is grown to the thickness of 30 angstroms. Then, the barrier layers andthe well layers are laminated in the order of the barrier+well+barrier+. . . +barrier layer. Thus, 11 barrier layers among which the initial 2layers are doped with Si and the remaining 9 layers are undoped and 10well layers among which the initial 2 layers are doped with Si and theremaining 8 layers are undoped are laminated alternately, in 21 layersin all, with the result that the active layer 7 in themulti-quantum-well structure having a total thickness of 3650 angstromsis obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.6V and the lightoutput power is 6.2 mW.

Example 8

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of GaN doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 200 angstroms using TMG, ammonia and silane gas.Subsequently, at 800° C., a well layer made of In_(0.3)Ga_(0.7)N dopedwith Si to 5×10¹⁷/cm³ is grown to the thickness of 30 angstroms usingTMG, TMI, ammonia and silane gas. Further, a barrier layer made ofundoped GaN is grown to the thickness of 200 angstroms and a well layermade of undoped In_(0.3)Ga_(0.7)N is grown to the thickness of 30angstroms. Then, the barrier layers and the well layers are laminated inthe order of the barrier+well+barrier+ . . . +barrier layer. Thus, 11barrier layers among which the initial layer is doped with Si and theremaining 9 layers are undoped and 10 well layers among which theinitial 1 layer is doped with Si and the remaining 8 layers are undopedare laminated alternately, in 21 layers in all, with the result that theactive layer 7 in the multi-quantum-well structure having a totalthickness of 3650 angstroms is obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.6V and the lightoutput power is 6.0 mW.

Example 9

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of GaN doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 200 angstroms using TMG, ammonia and silane gas.Subsequently, at 800° C., a well layer made of undoped In_(0.3)Ga_(0.7)Nis grown to the thickness of 30 angstroms using TMG, TMI and ammonia.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms, with the result that the active layer 7 in themulti-quantum-well structure is obtained.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.4V and the lightoutput power is 5.6 mW.

Example 10

Each layer down to the n-type contact layer 4 is formed in the samemanner as in Example 1.

(Second Undoped GaN Layer 5)

Next, the silane gas is stopped and at 1050° C., in the same manner, thesecond undoped GaN layer 5 is grown to the thickness of 1500 angstroms.

(n-Type Multi-layered Film 6)

Next, at 800° C., using TMG, TMI and ammonia, a second nitridesemiconductor layer made of undoped In_(0.03)Ga_(0.97)N is grown to thethickness of 20 angstroms. Subsequently, the temperature is raised and afirst nitride semiconductor layer made of undoped GaN is grown to thethickness of 40 angstroms. These operations are repeated to laminatealternately the layers in the order of the second layer+the first layer,in 10 layers, respectively. Finally, the second nitride semiconductorlayer made of GaN is grown to the thickness of 40 angstroms. Thus, ann-type multilayered film 6 in the super lattice structure is grown tothe thickness of 640 angstroms.

The active layer 7 and the remaining layers below the active layer 7 areformed in the same manner as in Example 1 to fabricate a LED device.Thus, there are provided the said n-side first multi-layered film 5 andthe n-side second multi-layered film 6 between the contact layer and theactive layer, with the result that the withstand static voltage isfurther improved.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.6V and the lightoutput power is 6.5 mW.

Example 11

The LED device is fabricated in the same manner as in Example 1, exceptthat the second undoped GaN layer 5 and the n-type multi-layered film 6are omitted.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.8V and the lightoutput power is 6.2 mW.

Example 12

The LED device is fabricated in the same manner as in Example 1, exceptthat the layer as will be described in the following part is formedbetween the p-type multi-layered film 8 and the p-type contact layer 9.

(p-Type Undoped AlGaN Layer)

After formation of the p-type multi-layered film, an undopedAl_(0.05)Ga_(0.95)N layer is grown to the thickness of 2000 angstroms.This layer contains a p-type impurity due to the diffusion of Mg fromthe p-type multi-layered film 8 and shows a p-type conductivity.

For the resulting LED device, the blue emission at a wavelength of 470nm is observed at the forward voltage of 20 mA. Vf is 3.4V and the lightoutput power is 6.5 mW.

Example 13

The LED device is fabricated in the same manner as in Example 3, exceptthat the active layer 7 is formed in the following manner.

(Active Layer 7)

A barrier layer made of undoped GaN is grown to the thickness of 200angstroms using TMG and ammonia. Subsequently, at 800° C., a well layermade of In_(0.35)Ga_(0.65)N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms and a well layer made of undoped In_(0.35)Ga_(0.65)N isgrown to the thickness of 30 angstroms. Then, the barrier layers and thewell layers are laminated in the order of the barrier+well+barrier+ . .. +barrier layer. Thus, 6 undoped barrier layers and 5 well layers amongwhich the initial 1 layer is doped with Si and the remaining 4 layersare undoped are laminated alternately, in 11 layers in all, with theresult that the active layer 7 in the multi-quantum-well structurehaving a total thickness of 1350 angstroms is obtained.

For the resulting LED device, the blue-green emission at a wavelength of500 nm is observed at the forward voltage of 20 mA. Vf is 3.8V and thelight output power is 5.2 mW.

Example 14

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active layer 7)

A barrier layer made of undoped GaN is grown to the thickness of 200angstroms using TMG and ammonia. Subsequently, at 800° C., a well layermade of In_(0.40)Ga_(0.60)N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms and a well layer made of undoped In_(0.40)Ga_(0.60)N isgrown to the thickness of 30 angstroms. Then, the barrier layers and thewell layers are laminated in the order of the barrier+well+barrier+ . .. +barrier layer. Thus, 5 undoped barrier layers and 4 well layers amongwhich the initial 1 layer is doped with Si and the remaining 3 layersare undoped are laminated alternately, in 9 layers in all, with theresult that the active layer 7 in the multi-quantum-well structurehaving a total thickness of 1120 angstroms is obtained.

For the resulting LED device, the blue-green emission at a wavelength of530 nm is observed at the forward voltage of 20 mA. Vf is 3.5V and thelight output power is 3.6 mW.

Example 15

The LED device is fabricated in the same manner as in Example 1, exceptthat the second n-type multi-layered film 6 is omitted. The resultingLED device showed a little worse device characteristics and a lowerlight output power as compared with in Example 1. But the light outputpower thereof is better than that of the conventional LED device.

Example 16

The LED device is fabricated in the same manner as in Example 1, exceptthat The thickness of the buffer layer 2 is 150 angstroms and thethickness of the first undoped GaN layer 3 is 1.5 μm. The similarresults to those in Example 1 are obtained.

Example 17

The LED device is fabricated in the same manner as in Example 13, exceptthat The thickness of the buffer layer 2 is 150 angstroms and thethickness of the first undoped GaN layer 3 is 1.5 μm. The similarresults to those in Example 13 are obtained.

Example 18

The LED device is fabricated in the same manner as in Example 1, exceptthat the active layer 7 is formed in the following manner.

(Active layer 7)

A barrier layer made of undoped GaN is grown to the thickness of 200angstroms using TMG and ammonia. Subsequently, at 800° C., a well layermade of In_(0.30)Ga_(0.70)N doped with Si to 5×10¹⁷/cm³ is grown to thethickness of 30 angstroms using TMG, TMI, ammonia and silane gas.Further, a barrier layer made of undoped GaN is grown to the thicknessof 200 angstroms and a well layer made of In_(0.30)Ga_(0.70)N doped withSi to 5×10¹⁷/cm³ is grown to the thickness of 30 angstroms. Furthermore,a barrier layer made of undoped GaN is grown to the thickness of 200angstroms and a well layer made of In_(0.30)Ga_(0.70)N doped with Si to5×10¹⁷/cm³ is grown to the thickness of 30 angstroms. Then, the barrierlayers and the well layers are laminated in the order of thebarrier+well+barrier+ . . . +barrier layer. Thus, 16 undoped barrierlayers and 15 well layers among which the initial 3 layers are dopedwith Si and the remaining 12 layers are undoped are laminatedalternately, in 31 layers in all, with the result that the active layer7 in the multi-quantum-well structure having a total thickness of 3650angstroms is obtained.

Thus, the active layer is formed in such a structure that the fartherthe Si doped layer is from the n-type layer, the smaller the amount ofdoped Si is in said layer, with the result that the similar results tothose in Example 1 are obtained.

Industrial Applicability

According to the present invention, the layers in the n-type layerregion of the active layer in the multiquantum-well structure composedof a well layer and a barrier layer are doped with Si and the dopedlayers are limited. The supply of the donor from the n-type layer can becompensated for, resulting in the nitride semiconductor device having ahigh light output power. Therefore, the nitride semiconductor devicesaccording to the present invention can be applied effectively to notonly light emitting devices such as light emitting diodes (LEDs) andlaser diodes (LDs), but also solar cells, light receiving devices suchas optical sensors and electronic devices such as transistors and powerdevices.

What is claimed is:
 1. A nitride semiconductor device which comprises anactive layer containing an n-type impurity and comprising a quantum welllayer or layers and a barrier layer or layers between n-type nitridesemiconductor layers and p-type nitride semiconductor layers, wherein atleast said quantum well layer at the proximate side in said active layerto said n-type nitride semiconductor layers is doped with an n-typeimpurity and wherein at least said quantum well layer at the proximateside in said active layer to said p-type nitride semiconductor layers isnot doped with an n-type impurity.
 2. A nitride semiconductor deviceaccording to claim 1, wherein said active layer is a MQW structurehaving (i) laminated layers and at least one of 1st to j-th layerscounting from the side proximate to said n-type nitride semiconductorlayers is doped with n-type impurity; wherein j′=i/6+2 where i is aninteger of at least 4, and wherein j is the integer portion of j′.
 3. Anitride semiconductor device according to claim 1, wherein said activelayer contains an n-type impurity and comprises an MQW structurecomprising quantum well layers and barrier layers between n-type nitridesemiconductor layers and p-type nitride semiconductor layers, wherein atleast said quantum well layer at the proximate side in said active layerto said n-type nitride semiconductor layers is doped with an n-typeimpurity and at least said quantum well layer at the proximate side insaid active layer to said n-type nitride semiconductor layers is dopedwith an n-type impurity, and wherein said barrier layer and/or saidquantum well layer at the proximate side to said p-type semiconductorlayers are not doped with n-type impurity.
 4. A nitride semiconductordevice according to claim 1, wherein said active layer contains ann-type impurity and comprises an MQW structure comprising quantum welllayers and barrier layers between n-type nitride semiconductor layersand p-type nitride semiconductor layers, wherein at least said quantumwell layer and said barrier layer at the proximate side in said activelayer to said n-type nitride semiconductor layers are doped with n-typeimpurity and wherein at least said quantum well layer and said barrierlayer at the proximate side in said active layer to said p-type nitridesemiconductor layers are not doped with n-type impurity.
 5. A nitridesemiconductor device according to claim 1, wherein said active layer isa MQW structure comprising a quantum well layer and barrier layer pairssandwiching said quantum well layer, wherein said barrier layer at theproximate side to said n-type nitride semiconductor layers are dopedwith n-type impurity, and said quantum well layer and said barrier layerat the proximate side to said p-type semiconductor nitride layers arenot doped with n-type impurity.
 6. A nitride semiconductor deviceaccording to claim 1, wherein said active layer comprises 9 to 15layers, at most 4 layers of which, counting from the proximate side tosaid n-type semiconductor layers, are doped with n-type impurity.
 7. Anitride semiconductor device according to claim 1, wherein said quantumwell layers in said active layer comprises In_(x)Ga_(1−x)N (0<x<1) whichis able to emit or receive a peak wavelength belonging to a range of 470to 530 nm.
 8. A nitride semiconductor device according to claim 1,wherein said n-type impurity is selected from the group consisting ofSi, Ge and Sn.
 9. A nitride semiconductor device according to claim 8,wherein said n-type impurity is Si.
 10. A nitride semiconductor deviceaccording to claim 1, wherein said n-type impurity content of saidactive layer is lower than that of said n-type nitride semiconductorlayers.
 11. A nitride semiconductor device according to claim 1, whereinthe n-type impurity content of the active layer decreases as thedistance from said n-type nitride semiconductor layers increases.
 12. Anitride semiconductor device according to claim 1, wherein the n-typeimpurity content of said active layer is in a range of 5×10¹⁶ to2×10¹⁸/cm³.
 13. A nitride semiconductor device according to claim 12,wherein the n-type impurity content of said barrier layer in said activelayer is in a range of 5×10¹⁶ to 2×10¹⁸/cm³.
 14. A nitride semiconductordevice according to claim 12, wherein the n-type impurity content ofsaid quantum well layer in said active layer is in a range of 5×10¹⁶ to2×10¹⁸/cm³.
 15. A nitride semiconductor device according to claim 9,wherein the n-type impurity content of said barrier layer in said activelayer is in a range of 5×10¹⁶ to 2×10¹⁸/cm³, whereas the n-type impuritycontent of said quantum well layer in said active layer is in a range of5×10¹⁶ to 2×10¹⁸/cm³ and lower than that of said barrier layer.
 16. Anitride semiconductor device according to claim 9, wherein the n-typeimpurity content of said barrier layer in said active layer is in arange of 5×10^(16 to) 2×10¹⁸/cm³, whereas the n-type impurity content ofsaid quantum well layer in said active layer is less than 5×10¹⁶ to2×10¹⁸/cm³ and lower than that of said barrier layer.
 17. A nitridesemiconductor device according to claim 2, wherein the thickness of saidbarrier layer or quantum well layer close or proximate to said n-typesemiconductor layers is larger than that of said barrier layer orquantum well layer close or proximate to said p-type semiconductorlayers.
 18. A nitride semiconductor device according to claim 2, whereinthe thickness of said barrier layer or quantum well layer close orproximate to said n-type semiconductor layers is smaller than that ofsaid barrier layer or quantum well layer close or proximate to saidp-type semiconductor layers.
 19. A nitride semiconductor emitting devicewhich comprises an active layer comprising quantum well layer or layersand barrier layer or layers between n-type nitride semiconductor layersand p-type nitride semiconductor layers, wherein said quantum layer insaid active layer comprises InxGa1−xN (0<x<1) having a peak wavelengthof 450 to 540 nm and said active layer comprises laminating layers of 9to 13, in which at most 3 layers from the side of said n-type nitridesemiconductor layers are doped with an n-type impurity selected from thegroup consisting of Si, Ge and Sn at a range of 5×10¹⁶ to 2×10¹⁸/cm³,and the other layers are not doped with an n-type impurity.
 20. Anitride semiconductor emitting device according to claim 19, wherein thethickness of said barrier layer or quantum well layer close or proximateto said n-type semiconductor layers is larger than that of said barrierlayer or quantum well layer proximate to said p-type semiconductorlayers.
 21. A nitride semiconductor emitting device according to claim19, wherein the thickness of said barrier layer or quantum well layerclose or proximate to said n-type semiconductor layers is smaller thanthat of said barrier layer or quantum well layer close or proximate tosaid p-type semiconductor layers.
 22. A nitride semiconductor emittingdevice according to claim 19, wherein said n-type impurity is Si.
 23. Anitride semiconductor emitting device according to claim 19, whereinsaid quantum well layer in said active layer comprises In_(x)Ga_(1−x)N(0<x<1) having a peak wavelength of 490 to 510 nm.
 24. A nitridesemiconductor emitting device according to claim 23, wherein saidbarrier layer in said active layer comprises In_(y)Ga_(1−y)N (0≦y<1,y<x).
 25. A nitride semiconductor emitting device according to claim 19,wherein said active layer comprises an MQW of In_(x)Ga_(1−x)N(0<x<1)/In_(y)Ga_(1−y)N (0≦y<1, y<x) lamination and is formed on ann-type multi-layer.
 26. A nitride semiconductor emitting deviceaccording to claim 25, wherein said multi-layer is a buffer superlattice layer undoped with n-type impurity and comprisingIn_(z)Ga_(1−z)N (0<z<1)/GaN lamination or Al_(w)Ga_(1−w)N (0<w<1)/GaNlamination.
 27. A nitride semiconductor emitting device according toclaim 26, wherein said GaN layer of said buffer super lattice layer hasa thickness of less than 70 Å and said barrier layer of said activelayer has a thickness of more than 70 Å.
 28. A nitride semiconductoremitting device according to claim 27, wherein said multi-layer aredoped with n-impurity and comprises lamination of GaN layer and a layerselected from the group consisting of a In_(z)Ga_(1−z) (0<z<1, z<y)layer having a larger band gap energy than that of said quantum welllayer and a Al_(w)Ga_(w)N (0<w<1) layer.
 29. A nitride semiconductoremitting device according to claim 28, wherein said n-type impurity fordoping into said active layer and said n-type clad layer is Si and theSi content of said active layer is in a range of 5×10¹⁶ to2×10¹⁸/cm³whereas the Si content of said n-clad layer is in a range ofmore than 5×10¹⁷/cm³ and larger than that of said active layer.
 30. Anitride semiconductor emitting device according to claim 19, wherein thefirst layer form the side of said n type nitride semiconductor layers isdoped with the n type impurity and the other layers are not doped withthe n type impurity.
 31. A nitride semiconductor emitting deviceaccording to claim 6, wherein the first and second layers from the sideof said n type nitride semiconductor layers are doped with the n typeimpurity.